Module B1 Upgrade: Model answers 6 © Oxford University Press 2011 Genes Higher tier question Click...

Module B1 Upgrade: Model answers 6

© Oxford University Press 2011

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Question

Explanation of question

Answer

Question (part 1)

(a) For each of the following descriptions, identify which one statement is not correct.

(i) Genes are A DNA molecules which controls how to make proteins B long strands of protein found in the nucleus C contained in chromosomes D found in the cell nucleus E copied when a cell divides.

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Question (part 2)(ii) Chromosomes are

A made partly of DNA B usually coiled up tightly C in the cell nucleus D in pairs in sperms and eggs E found in different numbers in different species.

(iii) Proteins are A found only in the cell nucleus B the molecules of which enzymes are made C either structural or functional D an important part of our food E made using instructions in DNA.

[1]

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Question (part 3)

(b) A couple have three sons. They would like to have a daughter. They think they are more likely to have a girl after three boys.

(i) Explain why the chance of their next child being a boy is 50%.

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(ii) State the probability that if the couple had two more children, they would

both be boys.

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Answer


In the multiple choice question (a), you are asked to spot the incorrect statement in each case. This tests your knowledge of genes covered in module B1 You and your genes.

In (b) (i), your understanding of the inheritance of sex in humans is tested. You need to recall that human males have XY sex chromosomes and females have XX sex chromosomes.

For (b) (ii), you need an understanding of the probabilities of inheritance, and will need to do some maths.



Question


Answer

D–C answer

(a) (i) E

(ii) D

(iii) A

(b) (i) Sperms are X or Y in equal numbers so the chances are 50/50 of having a boy or a girl.

comment

For (a) (i), E is incorrect. Genes are copied when a cell divides, giving a copy of each gene to each of the two new cells.

This response to (b) (i) is correct but the candidate needs to say more about what it means for sperm to be ‘X’ and ‘Y’, and why this affects the sex of the child.

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Answer

D–C answer (continued)

(ii) Chance is 50% + 50% = 100%

comment

This response to (b) (ii) is not correct. The probabilities have been added when they should have been multiplied.

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Question


Answer

B–A* answer

(a) (i) B

(ii) D

(iii) A

comment

The candidate has correctly identified the incorrect statements in (a).

In (i), genes are made of DNA, not protein.

In (ii), chromosomes are single in sperms and eggs, and become pairs again at fertilization.

In (iii), proteins are not found only in the nucleus. The cytoplasm contains a lot of proteins.

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Question


Answer

B–A* answer (continued)

(b) (i) Men have XY sex chromosomes. Half the sperm have an X and half a Y. Women are XX so all the eggs are X. There is a 50% chance that a Y sperm will fertilize an egg and make a boy. The previous children’s sex makes no difference.

(ii) The chance is 50% x 50% = 25%

comment

This answer to (b) (i) correctly explains why the chance of any child being a boy is 50%, and also states correctly that the sex of any previous children makes no difference to this chance.

This answer to (b) (ii) is correct. To find the probability of two events both happening, you need to multiply the two probabilities together.

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Module B1 Upgrade: Model answers 6 © Oxford University Press 2011 Genes Higher tier question Click...

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